1. Field of the Invention
The present invention relates to a valve system for an internal combustion engine, which is capable of driving intake valves and exhaust valves of the internal combustion engine to open and close at different timings.
2. Description of Related Art
In recent years, a valve system, which is capable of optimizing operating characteristics (e.g., opening/closing timing, an opening time period, and so forth) of intake valves and exhaust valves (engine valves) of a reciprocating internal combustion engine according to engine load and engine speed, has been developed and put into practical use.
To optimize the operating characteristics in this valve system, a mechanism has been developed which is capable of opening and closing the engine valves by selectively using a low-speed cam with a cam profile suitable for low-speed engine revolution or a high-speed cam with a cam profile suitable for high-speed engine revolution according to the revolutionary state of the engine. For example, there is known a mechanism which selectively uses a low-speed cam or a high-speed cam by using means for selectively supplying or releasing oil pressure. Further, a variety of techniques have been developed in which accumulating means is provided in a channel via which pressurized oil is supplied and released.
In Japanese Laid-Open Patent Publication (Kokai) No. 11-13429 and Japanese Laid-Open Patent Publication (Kokai) No. 10-54215, conventional valve systems are disclosed in which an accumulating means is provided. Conventionally, the accumulating means has been provided as a hydraulic control system and formed integrally with a variable valve control mechanism as disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 5-240012. The hydraulic control system is fastened within a cylinder head via bolts. However, if the accumulating means is disposed as disclosed in the third Patent publication, the accumulating means is connected to the cylinder head via another member. Therefore, the hydraulic control system is large in size, and the internal arrangement (layout) of the cylinder head is complicated. Also, a channel for pressurized oil needs to have an extra length so that the accumulating means can be connected to the cylinder head via another member. Further, oil leakage or the like occurring on a surface of abutment between another member and a mounting part of the cylinder head must be taken into consideration. As a result, the manufacturing cost is increased. On the other hand, in the case where the accumulating means is provided in a cylinder block, if an accumulating chamber is formed in the cylinder block and the accumulating means is formed integrally with the cylinder head, the accumulating chamber is integrated with the cylinder block, so that the manufacturing cost and the machining cost can be suppressed. However, the accumulating means is configured as part of an internal combustion engine, and hence the replacement and maintenance of the accumulating means are difficult.